Poster at the 40th annual meeting of the Biopysical Society . Abstract: Biophys. J. 70, A210 Multiphoton fluoroscence cross sections of the amino acid tryptophan and neurotransmitters serotonin and dopamine have been measured as a function of wavelength in the near infrared using a modelocked Ti:sapphire laser. The measured three photon fluorescence excitation spectra differ from the corresponding one-photon fluorescence excitation spectra. For the three photon processes, the detected fluorescence is observed to be proportional to the cube of the applied power at low power levels. At the highest intensities, the fluorescence deviates above or below the cubic values, depending on the excitation wavelength. These deviations reflect four photon excitation, photobleaching and other photochemical pathways currently under investigation. The magnitudes of the fluorescence signals predict detectable two and three photon fluorescence from large neurotransmitter vesicles. Combined with the inherent three dimensional resolution of multiphoton excitation using highly focused fermtosecond laser pulses and relative transparency of biological materialsto these longer wavelengths , these results suggest new imaging possibilities utilizing the native UV-excitable fluorescence of cellular materials